The old method of converting tungsten bearing material to sodium tungstate by means of an oxidizing fusion consists of melting the entire batch requirement of sodium nitrate and other materials together at one time, and gradually adding the tungsten bearing material to control the reaction rate until all the tungsten bearing material has been added and reacted. The fused mixture now containing sodium tungstate is then poured out into a suitable container for solidification or may be poured directly into water. Subsequent operations include removal, breakup, dissolution of the fusion cake in water, and further chemical processing. This method is used for both tungsten metal and tungsten carbide bearing material.
An alternate method is used for tungsten carbide containing material in which all the reactants are mixed together, and the reaction is initiated at one place by heating with a torch to form a localized melt. The reaction that results produces enough heat to cause the molten area and the reaction to spread to the entire batch without any further application of heat. The reaction rate is controlled by incorporating into the mixture either a sufficient quantity of "hard" tungsten carbide bearing material, which is slow to react, or sodium carbonate which dilutes the reactants and absorbs part of the heat produced by the reaction. Before the reaction mass cools and starts to solidify, it is poured out of the fusion vessel into a tray-type container for it to solidify into a relatively thin sheet.
These techniques have disadvantages since controlling the reaction rate is difficult. In an uncontrolled reaction, a large part of the molten mixture may expand due to rapid gas generation and overflow the fusion vessel. This problem arises primarily because of the thermal decomposition characteristics of the sodium nitrate or sodium nitrite in the fused mixture. All or part of the free oxygen is consumed in oxidation of the tungsten material depending on the composition and on the presence of other compounds. This oxidation is highly exothermic, producing enough heat to cause a rapid temperature rise in the fused mixture under certain conditions. A higher temperature causes the sodium nitrate to decompose more rapidly, leading to a rapid generation of nitrogen and oxygen, and, if sodium hydroxide is present, water vapor. These gases must be released to the atmosphere, but if they are generated too rapidly they push the fusion mixture out of the vessel.
Various methods of controlling the rapid generation of gases have been attempted with varying degrees of success or uncertainty. Even adding tungsten bearing material to the fused mixture at a rate to limit the reaction result in the rapid generation of gases described above.
The method of blending the entire charge of tungsten bearing material with sodium hydroxide and sodium nitrate (or sodium nitrite) relies on dilution with either tungsten bearing material or with sodium carbonate or both to moderate the rate. Sodium carbonate is undesirable because it raises the melting point of the mixture. If an excess is present, a higher temperature of the final mixture is required so it can be poured out of the fusion vessel, or else a larger amount of sodium hydroxide must be added to keep the melting point low enough for pouring. Controlling the reaction is also difficult due to variation in the charge of tungsten bearing material. There can be variations in the composition, surface area, or surface activity of the tungsten bearing material. This introduces uncertainty for each batch as to the rate of reaction. It is very difficult to control a rapid reaction.
Heretofore, prior processes for converting tungsten bearing material to sodium tungstate by a fusion of the mixture containing the entire batch charge of oxidizing reactant resulted in a reaction that is difficult to control.